For more than a century, gaseous anesthesia drugs have been utilized in anesthesia. An anesthesia gas which is liquid at room temperature and atmospheric pressure and is vaporized in an anesthesia vaporizer, is identified as being a volatile anesthetic agent. Gaseous anesthetic agents are also available.
Chloroform and ether were used as early volatile anesthetic agents and have been replaced with various further developments in the course of the past years for medical and safety reasons. Halothane (C---F.sub.3 --C--H--Cl--Br) was developed in the 1950's and is a present day anesthetic agent in general use. Other anesthetic agents in use today are Enflurane (C--F.sub.2 --H--O--C--F.sub.2 --C--F--Cl--H) and Isoflurane (C--F.sub.2 --H--O--C--H--Cl--C--F.sub.3) which have been developed in the 1980's.
Nitrous oxide (N.sub.2 O) is used for all anesthesia as a gaseous anesthetic agent and carrier gas. Nitrous oxide increases the effect of the anesthetic substances and thereby reduces their use. The inspiratory nitrous oxide concentration amounts usually to 70%. The concentrations of volatile anesthetic agents required for the anesthesia amount to between 1% and 2% in the inspiratory air of the patient. For a general anesthesia, the patient, as a rule, is ventilated with an anesthesia gas mixture of 70% N.sub.2 O, 30% oxygen and the particular volatile anesthetic agent. Halothane is metabolized by humans at 20%, enflurane at 2% and isoflurane at 0.2%. Nitrous oxide is exhaled unchanged.
In modern anesthesia ventilating apparatus, it is necessary for technical reasons to meter a far greater quantity of gas to the patient than is justified by the actual consumption. The excess gas is conducted into the anesthesia gas removal line. A possibility of reducing the consumption of anesthesia gases is the low-flow anesthesia. The gas flow in the low-flow anesthesia amounts to 1/3 to 1/6of that which is conventional in modern gas anesthesia. In order to be able to control the metering of gas with adequate precision, the low-flow anesthesia requires a ventilating apparatus which has a more complicated configuration than those apparatus which have been utilized up to now. For example, they must be tighter and be made with greater precision in order to meter the required gas concentration for anesthesia with adequate precision. Low-flow anesthesias are basically possible with the technical possibilities provided today and with the introduction of a new generation of anesthesia ventilating apparatus; however, this cannot be utilized in all surgery.
It is known since the 1940's that the rare gas xenon is suitable as an inhalation anesthetic agent. Xenon affords many advantages medically. Advantages have been found when compared to nitrous oxide in investigations which have been carried out utilizing xenon as an inhalation anesthetic agent. These advantages include: a sleep which has been found to be very pleasant by the patient, higher circulatory stability, lack of release of stress hormones and advantages for the local circulation of individual organs. Xenon acts anesthetically stronger than nitrous oxide from which an additional anesthetic-agent saving effect results. Also, xenon is better suited than nitrous oxide and isoflurane for the low-flow anesthesia because of its lower blood/gas solubility and therefore builds up more rapidly and reduces more rapidly in the body. Also, xenon as a rare gas is with the greatest probability of no problem when considered in the context of environmental and workplace exposure aspects. For the above reasons, xenon is regarded as a virtually ideal anesthetic gas. The only disadvantage of xenon has been its high cost. For this reason, it has been pointed out in almost all investigations that xenon is only suitable for scientific purposes and cannot be considered for clinical use.
The use of xenon as an inhalation anesthetic agent would be a significant advance in the area of medical applications and in the exposure to the environment and workplace. The high cost of xenon, however, makes an adequate recovery method absolutely necessary.
Published European patent application 0,287,068 discloses an anesthesia apparatus for ventilating a patient with xenon as a constituent of the respiratory gas. The respiratory gas here comprises a gas mixture of oxygen and xenon and is supplied to the patient via an inspiratory line and is conducted back to the anesthesia apparatus via an expiratory line. A cleansing unit is disposed in the expiration line and includes a filter for the adsorption of carbon dioxide and water. The cleansing unit also has an active charcoal filter. The expirated respiratory gas which is so cleaned is pumped into two pressure vessels by a compressor operating as a pumping device. The pressure vessels, in addition to oxygen, contain either a concentration of xenon less than 80% or greater than 80%. The expirated respiratory gas is analyzed with respect to its xenon content and, depending upon the measured xenon concentration, is supplied to one or the other pressure vessel. The pressure vessels are connected to the gas mixer of the anesthesia apparatus and function as a reservoir for the inspirated respiratory gas supplied to the patient.
Although the known anesthesia apparatus permits a substantial recovery of xenon from the expiratory respiratory gas one is, however, dependent upon the instantaneous gas-type composition present in the pressure vessels when adjusting the inspiratory xenon concentration. This composition can change continuously because of the continuous return of the expiratory respiratory gas. A continuous control of the xenon concentration, which is present in the pressure vessels, is necessary and this makes the manipulation of the apparatus overall difficult. Furthermore, xenon is lost because of the purging operations within the expiratory line. This xenon cannot be returned to the two pressure vessels and must be separately processed.
Published German patent application 3,000,191 discloses a method for recovering xenon from exhaled respiratory gas. This method is based on the adsorption of xenon in heated active charcoal beds with a subsequent desorption utilizing a purging gas. It is disadvantageous in this method that only a portion of the inhaled xenon volume can be recovered and that an application in the clinical routine operation is not possible because of the high complexity of the apparatus.